1. Field of the Invention
The invention relates to a brushless motor used as a main motor for simultaneously driving various mechanisms such as a copying machine and a laser beam printer. More particularly, the invention relates to a brushless motor including a stator core (iron core) around which a stator coil is wound, a rotor attached with a ring-shaped magnet, a Hall element for detecting the position of the rotor, and a speed detector for detecting the rotating speed of the rotor, and integrally incorporating a drive/control circuit for rotatably driving and controlling the rotor and a motor.
2. Description of the Related Art
Recently, document products such as a copying machine and a laser beam printer are oriented to have high image quality, high-speed printing and coloring, and to simultaneously realize such, tend to use a so-called tandem system that includes a photosensitive drum for each color. The conventional configuration including one drum now is a configuration including two drums or four drums, and thus the apparatus unavoidably becomes larger. To suppress such enlargement, however, the main motor for driving various mechanisms is required to be thinner, smaller, to save as much space as possible, and to have high output.
The brushless motor integrally incorporating a motor section including a stator equipped with a stator core (iron core) suitable for high output, and a drive/control circuit for rotatably driving and controlling the motor is generally used as the main motor, and the entire motor including the circuit section is required to be thinner, smaller, to save as much space as possible, and to have high output.
To this end, the outside dimension of the stator part of the motor section and the circuit section is desirably made as close as possible to the dimension of a square circumscribing the rotor part, and similarly, the height dimension is desirably made as close as possible to the height dimension of the rotor part.
Conventionally, the well-known brushless motor including the stator core (iron core) includes a rotor in which a ring-shaped magnet having a plurality of magnetic poles facing the stator core (iron core) is provided on the inner peripheral surface of the rotor yoke and a stator assembly in which the stator coil (armature coil) is wound around each stator core (iron core). The stator assembly is attached to a circuit substrate (stator base) equipped with a Hall element (position detecting element) and a drive circuit for rotatably driving the rotor by way of a housing (bearing holder). The rotating position of the rotor is detected by detecting the magnetic pole of the magnet of the rotor with the Hall element (position detecting element), and from the drive circuit to where the output signal of the Hall element is input, the drive current controlled in accordance with the output signal of the Hall element (position detecting element) is supplied to the stator coil (armature coil). The stator coil (armature coil) then generates a magnetic field corresponding to the rotating position of the rotor. The rotor is configured (hereinafter referred to as a first conventional art) so that a continuous rotating force is generated by the interaction of the magnetic field generated by the stator coil (armature coil) and the magnetic pole of the magnet (refer to e.g., JP-A 8-88964 (1996).).
FIG. 6 shows a configuration of a conventional brushless motor.
In FIG. 6, reference character 1 refers to a shaft, reference character 2 refers to a rotor yoke, reference character 3 refers to a magnet, reference character 4 refers to a stator coil (armature coil), reference character 5 refers to a stator core (iron core), reference character 6 refers to a Hall element (position detecting element), reference character 7 refers to a drive circuit, reference character 8 refers to a circuit substrate (stator base), reference character 9 refers to a bearing, and reference character 10 refers to a housing (bearing holder).
In the conventional brushless motor, the Hall element 6 is arranged at a position where the magnetic flux of the magnet 3 of the rotor is easily picked up, that is, on the inner diameter side of the magnet 3 on the surface side facing the magnet 3 of the circuit substrate (stator base) 8 to obtain the output necessary for position detection from the Hall element (position detecting element) 6. When arranged at such position, however, the Hall element (position detecting element) 6 is also arranged close to the stator coil (armature coil) 4, and thus the magnetic field generated by the excitation of the stator coil (armature coil) 4 influences the Hall element 6 as a noise, and a stable position detection may not be performed.
As high output is required, particularly, in the brushless motor used as a so-called main motor for simultaneously driving various mechanisms such as a copying machine and a laser beam printer, a large amount of current must flow to the stator coil (armature coil) 4. If the drive current is increased, however, the magnetic field generated at the stator coil (armature coil) 4 becomes large, which generated magnetic field influences the Hall element 6, thereby making a stable position detection difficult.
As shown in FIG. 7, a solution for the above problem includes a technique (hereinafter referred to as a second conventional art) for preventing the magnetic field generated by the excitation of the stator coil (winding wire) 4 from influencing the Hall element (detecting element) as noise with a configuration in which one portion of the magnet 3 arranged in the rotor yoke 2 is exposed from the rotor yoke 2, the Hall element (detecting element) 6 is arranged exterior to the magnet 3 in correspondence to the exposed portion of the magnet 3, and the leakage flux of the exposed portion of the magnet is detected (refer to e.g., JP-A 8-172763 (1996)).
Further, a solution different from the second conventional art includes a technique (hereinafter referred to as a third conventional art) in which the magnetic field generated by the excitation of the stator coil (driving coil) 4 is assumed to influence the output waveform of the Hall element (position detecting element) 6 thus producing deformation at a zero cross point of the output waveform, the deformation being produced in a direction that delays the current feed switching timing toward the stator coil (driving coil) 4, and as shown in FIGS. 8 and 9, a driving magnet 3a and a position detecting magnet 3b are arranged as the magnet (driving magnet) 3 of the rotor in such a way that the boundaries where the magnetic poles are opposite poles with respect to each other contact, thereby eliminating the delay of current feed switching by inversing the output of the Hall element (position detecting element) 6 so that the deformation of the output waveform is produced in a direction that accelerates the current feed switching toward the stator coil (armature coil) 4 (refer to e.g., JP-A 7-327351 (1995)).
More specifically, as shown in FIG. 9, the driving magnet 3a and the position detecting magnet 3b are arranged on the magnet (driving magnet) 3 of the rotor in such a way that the boundaries where the magnetic poles are opposite poles with respect to each other contact. Further, the Hall element (position detecting element) 6 is arranged on the circuit substrate (substrate) 8 at a position facing both the position detecting magnet 3b and the stator core 5 with which the stator coil (driving coil) 4 is wound. The planar arrangement of the Hall element 6 is configured as shown in FIG. 10, where the Hall element (detecting element) 6U of U phase, the Hall element (detecting element) 6V of V phase, and the Hall element (detecting element) 6W of W phase are arranged in correspondence to the each teeth of the U phase, the V phase and the W phase of the stator core (iron core) 5. The arrow indicates the direction of rotation.
According to this configuration, the composite magnetic field of the magnetic field generated by the excitation of the stator coil (driving coil) 4 and the magnetic field of the position detecting magnet 3b is detected by the Hall element (position detecting element) 6, thereby eliminating the delay of current feed switching toward the stator coil (driving coil) 4.
FIGS. 11A to 11G (FIGS. 9A to 9G of JP-A 7-327351 (1995)) shows that the magnetic field of the U phase and the W phase generated by the excitation of the stator coil (winding wire) 4 influences the output waveform of the Hall element (detecting element) 6U of the U phase, thus producing deformation at the zero cross point of the output waveform. The deformation is produced in a direction that delays the current feed switching timing toward the stator coil (driving coil) 4. FIG. 11A is the generated magnetic field of the driving magnet toward the Hall element (detecting element) 6U, FIG. 11B is the generated magnetic fields of the U and W phase cores, FIG. 11C is the generated magnetic field of the U and W phase cores toward the Hall element (detecting element) 6U, FIG. 11D is the generated magnetic field of the composite cores of U and W phases toward the Hall element (detecting element) 6U, FIG. 11E is the output waveform of the Hall element (detecting element) 6U, FIG. 11F is an enlarged view of the horizontal axis of the main part of FIG. 11E, and FIG. 11G is the generated magnetic field of the actual W phase core.
FIGS. 12A to 12E (FIGS. 3A to 3E of JP-A 7-327351 (1995)) show that the output of the Hall element (position detecting element) is inversed and the deformation of the output waveform is produced in a direction that accelerates the current feed switching toward the stator coil (driving coil). FIGS. 12A and 12B show the magnetic field toward the Hall element (detecting element) 6U and the magnetic field toward the core, FIG. 12C shows an output waveform of the Hall element (detecting element) 6U, FIG. 12D shows an enlarged view of the horizontal axis of the main part of FIG. 12C, and FIG. 12E shows an enlarged view of the generated magnetic field of the W phase core.
In the brushless motor used as the main motor for simultaneously driving various mechanisms such as a copying machine and a laser beam printer, not only the rotating speed of the motor, but the rotating phase in relation with the various mechanism sections of the apparatus driven by way of an output shaft and a decelerating mechanism attached to the output shaft must also be accurately controlled. Thus, the brushless motor requires a speed detector having a certain degree of resolution.
The speed detector of the brushless motor suited for the above application includes a so-called pattern FG system. The ring-shaped FG magnet subjected to NS multi-pole magnetization along the circumferential direction is arranged on the rotor side, the FG pattern including the generator wire elements of the same number as the magnetized poles of the FG magnet connected in series along the circumferential direction is arranged on the stator side, and the speed detecting signal (FG signal) of the frequency proportional to the rotating number of the motor produced in the FG pattern by the rotation of the motor is obtained. When incorporating the speed detector of the FG pattern system in the brushless motor, a configuration in which the driving magnet of the motor and the FG magnet are integrated by performing magnetization for the driving magnet on the inner peripheral side of the rotor magnet and NS multi-pole magnetization for the FG magnet (hereinafter referred to as FG magnetization) on the end face side, and the FG pattern is formed on the circuit substrate of the motor is desirable as a configuration with no influence on the shape of the motor and no additional component.
However, in either of the second conventional art and the third conventional art, an adverse effect arises due to performing FG magnetization on the end face on the circuit substrate (substrate) side of the magnet (driving magnet) of the rotor.
That is, the Hall element (position detecting element) must be arranged in the vicinity of the end face on the circuit substrate (substrate) side of the magnet (driving magnet) of the rotor, and thus the influence of FG magnetization formed on the end face of the magnet (driving magnet) of the rotor cannot be avoided, and a stable position detection becomes difficult.
In the second conventional art, the influence of FG magnetization can be prevented by spacing the Hall element (position detecting element) away from the magnet (driving magnet) of the rotor to an extent the influence of FG magnetization can be neglected with the dimension of one portion of the magnet of the rotor exposed from the rotor yoke being sufficiently large to an extent the influence of the FG magnetization can be neglected, but when the exposed portion of the driving magnet is large, the leakage flux increases thus increasing loss or unnecessary magnetic noise. Alternatively, the influence of FG magnetization can be prevented by attaching the Hall element (position detecting element) so as to float from the surface of the circuit substrate (substrate) by a certain extent, but this may increase the attachment cost and the like of the Hall element (position detecting element).
The third conventional art acts counter to the technically necessary configuration, and thus the FG magnetization becomes impossible to be performed on the end face of the circuit substrate (substrate) side of the magnet (driving magnet) of the rotor.
As shown in FIG. 13, as a solution for the above conventional problem in the brushless motor used as a so-called main motor for simultaneously driving various mechanisms such as a copying machine and a laser beam printer, a technique (hereinafter referred to as fourth conventional art) for preventing the influence on the Hall element (position detecting element) as noise with a configuration in which the rotor magnet 3 is extended toward the circuit substrate (print substrate) 8 side, and the Hall element (position detecting element) 6 is arranged on the circuit substrate (print substrate) 8 so that the direction of magnetic sensitivity is in the radial direction at a position facing the extended portion to greatly reduce the components in the radial direction of the magnetic field (leakage flux) generated by the excitation of the stator coil 4 at the position of the Hall element (position detecting element) 6, is adopted (refer to e.g., JP-U 57-113681 (1982)).
In the fourth conventional art, the rotor magnet 3 is extended toward the circuit substrate (print substrate) 8 side, and thus the gap between the end face of the rotor magnet 3 and the circuit substrate (print substrate) 8 can be reduced. Therefore, by performing FG magnetization on the end face on the circuit substrate (print substrate) 8 side of the rotor magnet 3 and forming the FG pattern on the circuit substrate (print substrate), the magnetic flux from the FG magnet that interlinks with the FG pattern becomes large. That is, the configuration is suitable for incorporating the speed detector of FG pattern system. Further, with regard to the influence of the magnetic field generated by the FG magnet of the rotor magnet (driving magnet) 3, the stable position detection becomes possible since the component in the radial direction at the position of the Hall element (position detecting element) 6 is small.
The brushless motor adopting the fourth conventional art and integrally incorporating the drive/control circuit for rotatably driving and controlling the rotor with the motor is shown in for example, FIG. 14. A configuration (hereinafter referred to as fifth conventional art) for fixing, in addition to the rotor magnet (driving magnet) 3, the FG magnet (speed detecting magnet) 11 on the end face on the side of the circuit substrate 8 of the rotor magnet (driving magnet) 3, or on the upper end face of the driving magnet 3 herein, is proposed (refer to e.g., JP-U 7-9078 (1995)). Reference character 12 is the FG pattern (speed detecting flat coil) and is formed on the lower surface of the circuit substrate 8. The drive/control circuit for rotatably driving and controlling the rotor of a control integrated circuit 13 and the like is mounted on the upper surface of the circuit substrate 8. The speed detecting magnet 11 is a ring-shaped magnet and the N, S magnetic poles of the same number as the number of turns of the FG pattern (speed detecting flat coil) 12 are magnetized alternately along the circumferential direction on the upper end face of the speed detecting magnet 11. The FG pattern (speed detecting flat coil) 12 faces the upper end face of the speed detecting magnet 11. When the speed detecting magnet 11 rotates integral with the rotor yoke 2, the FG pattern (speed detecting flat coil) 12 is induced to the speed detecting magnet 11 to output the speed detecting signal or a signal having a frequency corresponding to the rotating speed of the rotor yoke 2. A space 14 is provided between the control integrated circuit 13 and the upper surface of the circuit substrate 8, and electronic chip components 15 are arranged in the space 14.
The Hall element 6 is arranged at the bottom surface side of the circuit substrate 8, and a plurality of them are arranged at a position close to the FG pattern (speed detecting flat coil) 12. Reference character 16 is a motor attachment plate having an area substantially covering the entire upper surface of the motor, and is arranged near the circuit substrate 8 so that the bottom surface contacts the upper surface of the control integrated circuit 13.
The control integrated circuit 13, the electronic chip components 15 and the like are arranged on the upper surface of the circuit substrate 8, as mentioned above. On the lower surface of the circuit substrate 8, the FG pattern (speed detecting flat coil) 12, a circuit pattern and the like is printed, and the Hall element 6 and the like is arranged, as mentioned above. An outer lead 17 of the control integrated circuit 13 is projected out toward the lower surface of the circuit substrate 8.
In the configuration of the fifth conventional art, although the chip components 15 are mounted between the control integrated circuit 13 and the circuit substrate 8 to reduce the outer diameter of the motor (circuit substrate), a gap must be formed between the control integrated circuit 13 and the chip components 15 and thus the motor is prevented from being thin.
The speed detector of FG pattern system has the FG pattern (speed detecting flat coil) 12 arranged so as to surround the vicinity of the stator core 5, and thus the noise generated from the stator coil 4 is easily picked up. Thus, a so-called cancel pattern for canceling the components produced by such noise must be arranged in parallel with the outer peripheral side or the inner peripheral side of the FG pattern (speed detecting flat coil) 12.
Particularly, as the PWM driving method is generally adopted to enhance the power supply efficiency of the motor in the above mentioned main motor, the noise level generated from the stator coil 4 becomes very large. Further, in the conventional brushless motor for the main motor, the driving method of supplying the drive current that generates the magnetic field, which has the waveform of FIG. 11B (FIG. 9B of JP-A 7-327351 (1995)) showing the third conventional art, from the stator core to the stator coil (winding wire) 4 is adopted, and thus a rapid magnetic field change occurs and becomes the cause of influence on the FG pattern. The cancel pattern for the noise is thus essential.
Generally, the speed detector of FG pattern system reduces the influence of run-out and the like of the rotor part (FG magnet) during rotation or eccentricity of the FG magnet of the rotor part, and thus the length in the radial direction of the generator wire elements of the FG pattern must be longer than the length in the radial direction of the FG magnet.
When reducing the outer diameter of the motor while satisfying the above essential conditions, in the configuration of the fifth conventional art, the Hall element 6 is arranged on the same side as the FG pattern 12 of the circuit substrate 8, and in order to secure the length of the generator wire elements of the FG pattern with the inner peripheral diameter of the FG pattern 12 or the inner peripheral diameter of the cancel pattern being a diameter sufficiently smaller than the inner peripheral diameter of the FG magnet 11, the installing position of the Hall element 6 must be moved toward the shaft center of the motor. The distance between the rotor magnet 3 and the Hall element 6 thus increases and a sufficient position detection signal is not obtained. In order to reduce the distance between the rotor magnet 3 and the Hall element 6, the FG pattern 12 and the cancel pattern 18 around the Hall element 6 must have a configuration so as to narrow the width, as shown in FIG. 15, or to circumvent toward the outer side of the shaft center of the motor. In this case, the length of the generator wire elements becomes uneven, and worsens the speed detecting precision.